For example, in the process for manufacturing a semiconductor device made of a semiconductor substrate such as silicon, gallium arsenide, or the like, multiple step operations are carried out for patterning the film, such as insulating film, semiconductor film, metal film, or the like, which is formed over the substrate, are carried out. Such film patterning is executed by forming resist patterns over the film as a mask, and then etching the film in areas that are not covered by the resist.
The resist patterns are formed through the processes of baking, exposure, development, and the like after the resist is coated over the substrate on which the film is formed. The coating of the resist is carried out by dispensing a liquid resist over the substrate, and spreading the liquid resist over an overall surface of the substrate by relying on the centrifugal force of rotation of the substrate and the wetting properties of the resist.
Japanese Patent Application (KOKAI) Sho-63-313160-A, Japanese Patent Application Publication (KOKAI) Hei-10-275761-A, and Japanese Patent Application Publication (KOKAI) Hei-11-260717-A disclose that the rotation speed should be changed in two or three stages, when the resist is spread over the substrate by placing resist over a rotating substrate.
Sho-63-313160-A discloses that a spin speed of a square-shaped substrate should be divided into two stages such that in the first stage, the number of rotations R1 is held for a time period T1, and then in the second stage, the number of rotations R2 is held for a time period T2, during the coating of the resist over the square-shaped substrate. The conditions of R1, R2 and T1 are set as R1≦2000 rpm, R2≦1,000 rpm, R1>R2, and R1×T1≦10,000 rpm·sec.
The reason why the number of rotations R1 in the first stage is set to 2,000 rpm or less is to prevent the drying of the resist liquid that was dispensed onto the substrate. The reason why the number of rotations R2 in the second stage is set to 1,000 rpm or less is to reduce the resulting variation in the thickness of the resist film at the edge of the square-shaped substrate and the thickness of the resist film at the center portion of the square-shaped substrate. In this case, T1 in the time required to accelerate and decelerate of the substrate during the first stage.
The conditions are same as above whether stopping or continuing the rotation of the square-shaped substrate during the first stage and the second stage.
In contrast, Hei-10-275761-A and Hei-11-260717-A set forth that the first rotation speed of the semiconductor wafer as a substrate onto which the resist is dispensed should be set to 3500 rpm or 4500 rpm respectively. Also, in the resist coating method employing such a first a rotation speed, the rotation speed of the semiconductor wafer should be changed successively in three stages as described in Hei-10-275761-A and Hei-11-260717-A.
Hei-10-275761-A sets forth that (i) the resist liquid should be dispensed onto a semiconductor wafer that is being rotated at a first rotation speed of 4500 rpm, (ii) then the rotation speed of the substrate should be reduced to a the second rotation speed immediately after stopping the dispensing of the resist, (iii) then this second rotation speed should be maintained for a predetermined time, and (iv) then the rotation speed should be increased to a third rotation speed in a range between the first rotation speed and the second rotation speed. Accordingly, the resist can be coated uniformly onto the substrate without leaving a wave pattern, or the like, over the semiconductor wafer surface, according to description of Hei-10-272761-A.
Also, Hei-11-260717-A sets forth a resist liquid coating method where a liquid agent such as a thinner, or the like, is dispensed onto the surface of a semiconductor wafer before the semiconductor wafer is rotated at a first rotation speed. Accordingly, the amount of resist liquid to be dispensed can be reduced and also the thickness of the resulting resist coating can be controlled uniformly, according to the description of Hei-11-260717-A.
Also, Hei-11-260717-A sets forth that a thinner should be dispensed over the semiconductor wafer before the resist is dropped onto the semiconductor wafer, the semiconductor wafer should then be rotated at 2,000 rpm for one second, the resist liquid then should be dispensed over the semiconductor wafer during the time at which the rotation of the semiconductor wafer is increased up to 3500 rpm, and then the rotation should be reduced to 2,000 rpm or 100 rpm. Here, FIGS. 11 and 12 of Hei-11-260717 compares the case in which the amount of liquid that is dispensed is set to 1.5 cc with the case in which the amount of dispensed resist liquid is 0.5 cc.
The case where the amount of dispensation of the resist liquid is set to 1.5 cc and the rotation speed of the semiconductor wafer is reduced to 2,000 rpm after the resist is dispensed is compared with the case where the amount of dispensation of the resist liquid is set to 0.5 cc and the rotation speed is reduced to 100 rpm after the resist is dispensed, with reference to FIGS. 11 and 12 in Hei-11-260717. In this case, the former conditions could result in better uniformity of the film thickness of the resist.
According to Hei-11-260717-A, it is appreciated that, in order to make a uniform thickness of the resist coating over the semiconductor wafer, a large amount of the resist liquid, such as 1.5 cc, should be supplied onto a semiconductor wafer rotating at 3500 rpm and then the rotation speed of the semiconductor wafer should be reduced not to about 100 rpm, but to about 2,000 rpm, which is a relatively high value after the resist liquid is dispensed.
Also, according to Hei-11-260717-A, in a first case where the amount of the resist liquid being supplied is reduced to 0.5 cc, when the rotation speed of the semiconductor wafer is reduced to 100 rpm after the resist liquid is dispensed, flatness of the resist film can be improved to some extent as compared to a second case where the rotation speed is reduced to 2,000 rpm. However, the flatness of the resulting resist of the first case is considerably inferior as compared to the case where the amount of dispensation of the resist is set to 1.5 cc.
Even though variations in the thickness of the resist coating resulting from dispensing 1.5 cc of the resist liquid can be reduced, the centrifugal force scatters more of the resist liquid off the semiconductor wafer than the resist liquid remaining over the semiconductor wafer. For example, FIG. 11 of Hei-11-260717 provides an example where 1.5 cc of resist liquid is applied to an 8-inch wafer, resulting in resist film having a relatively uniform thickness of about 7340 to 7360 Angstrom. Thus, the resist liquid scattered off the substrate is not used to coat the resist film again.
There is a need for an improved method of coating a substrate with a coating liquid such as resist, SOG, polyimide, or the like by rotating the substrate such that the coating liquid is spread over the substrate in a uniform thickness.